# Enrichment of viable bactoeria in a micro-volume by free-flow electrophoresis

This paper essentially presents a new way to enrich bacteria with free-flow electrophoresis so that you can have a macro to micro interface for microfluidic analysis of samples in clinical settings. The paper describes a little bit how they use phaseguides to set up the experiment, but that does not seem to be the focus, so relation the phaseguide paper is not overwhelming.

#### Free flow electrophoresis works as follows:

• bacteria suspended in buffer are injected in a continuous flow
• surface of bacteria contains among others carboxylate, phosphate, and amino groups that are negatively charged in a pH range of 6–9
• an electric field ${\vec E}$ is applied perpendicular to the flow direction
• a negatively charged bacterium entering the chamber is subjected to a drag-force in the y-direction and an electrophoretic induced force in the x-direction resulting in a deflection from the initial flow direction towards the anode
• Degree of deflection $\theta$ depends on relation of electrophoretic velocity ${\vec v_c}$ and the hydrodynamic velocity ${\vec v_h}$ of the bacterium
$$tan \theta = \frac{\vec v_c}{\vec v_h}$$
• Hydrodynamic velocity varies with position of bacteria in the chamber (parabolic flow profile)
• Electrophoretic velocity is the product of the electric field strength and electrophoretic mobility of the bacteria ($\mu$)
$${\vec v_c} = \mu {\vec E}$$
• Constant electrophoretic trapping at the gel interface can be obtained by keeping the deflection angle constant.
• Governed by:
$$tan \theta = \frac{\mu {\vec J}}{\sigma {\vec v_h}}$$ $${\vec J} = \sigma {\vec E}$$
• here $J$ is current density and $\sigma$ is the conductivity of the medium
• Picture!

#### Chip Design

• The diagram pretty much says it all:

#### Bacteria

• Used Gram-positive bacterial strain *Enterococcus faecalis Symbioflor 1
• Achieve $~10^6 \text{ CFU ml}^-1$

#### Results

• A 75% optimal increase in trapping is observed at 250 $\mu A$ current
• Some other results, but basically it worked
Written on October 13, 2014